1. Field
The present description pertains to the field of determining the location of a vehicle using a portable navigation device and, in particular, to correcting location determinations to compensate for the possible misalignment of the portable location device in a vehicle.
2. Related Art
With the development of radio and space technologies, several satellite based navigation systems have already been built and more will be in use in the near future. One example of such satellite based navigation systems is the Global Positioning System (GPS), which is built and operated by the United States Department of Defense. The system uses twenty-four or more satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. These satellites are placed in six different orbits such that at any one time as many as six satellites are visible at any one location on the surface of the earth (except near the Earth's poles). Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it. Using signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
GPS and other satellite based navigational systems have some limitations such as the availability of a sufficient number of satellite signals. Satellite signals are sometimes not available in deep canyons, in areas with large numbers of buildings blocking the direct satellite signals, and in dense forest areas. In addition to this, the satellite signals can be completely blocked or greatly attenuated inside buildings. In addition, tunnels and bridges can block satellite signals resulting in large jumps in the indicated position at the exit of the tunnel after new satellite signals are received. In addition to this, some parameters like the steering of the vehicle, etc cannot be measured using satellite signals. To reduce these errors, other complementary methods are often used with satellite navigational systems to prevent interruptions in the position information. Inertial sensors such as gyroscopes are used to measure changes in direction. Accelerometers are used to estimate the acceleration of the vehicle, both backwards and forwards and from side to side. A host of similar devices are used to improve the accuracy and the consistency of a navigation system.
The application of low-end MEMS inertial sensors for land vehicle navigation is becoming popular because of the significant cost reduction in these sensors. After the position of a vehicle is initially determined, the inertial sensors allow the position of the vehicle to be determined as the vehicle moves, even if the satellite signals are blocked. The determination of the position based on measuring the vehicle movement is known as dead reckoning. The accuracy of a dead reckoning position and how long it remains accurate depends on the quality of the sensors and how well they are calibrated. In some systems dead reckoning is also used to improve the accuracy of the satellite location determinations.
For built-in systems, inertial sensors are mounted on the vehicle in a fixed, known position, with good accuracy, so as to use some motion constraint of the vehicles, such as a non-holonomic constraint, to improve the navigation accuracy. This works well for satellite-based navigation systems that are permanently installed in a car, such as those offered by the manufacturer.
Personal navigation devices (PND) are often inserted into a cradle, placed on the dashboard or put in some other temporary location inside the vehicle. The position of the PND with respect to the vehicle can differ each time the PND is brought back into the vehicle. There also may be adjustment of the angles of its position. Assuming the PND is mounted so that its screen is visible to the driver, the misalignment between the device and the vehicle might be any angle between +/−45 degrees for yaw and +/−30 degrees for roll and pitch. If the PND does not have a cradle and does not have a display screen, then the misalignment can be greater.
In either case, estimating and compensating for the misalignment angles can greatly increase the accuracy of any navigation using dead reckoning. Estimating and compensating for misalignment is even more important for incomplete inertial sensor configurations, for example a heading gyro combined with a three-axis accelerometer. Incomplete systems are often used for vehicle navigation to further reduce the cost of the navigation system. In this case, the alignment between the inertial sensors and the vehicle becomes more important when the omitted sensors are replaced by assumptions about the vehicle's motion.